1. Field of the Invention
This invention relates to video data transmission systems. More particularly, the invention relates to systems and methods of in-service testing of compressed digital broadcast video.
2. Prior Art
In video transmission networks the Motion Picture Expert Group (MPEG) 2 compression algorithm and ATM networks can cause perturbations in video transmission signals that are not normally seen in an analog only network. To detect network anomalies and failures real-time, inservice circuit testing is performed so that service may be restored with minimum circuit outage. The technique of EIA/TIA 250 C in-service testing is well-known in the television industry. Broadcasters have historically used the Video Blanking Interval (VBI) lines 10-20 of both fields to insert test signals. Also embedded in the VBI are closed caption text and Society of Motion Picture and Television Engineers (SMPTE) time codes. The VBI is not part of the active video area and, therefore, is not seen by viewers.
MPEG-2 video encoders however preclude in-service testing because the encoder filters VBI lines 1 through 21 exclusive of each screen or frame in order to reduce the transmitted bandwidth. The video encoder copies the SMPTE time code into the Group Of Pictures (GOP) header and the closed caption text is passed as user data in the MPEG-2 transport stream. The test signals are ignored. At the receiving end, the VBI lines are regenerated by the MPEG-2 decoder and the SMPTE time codes and closed captions text are reinserted, however, the VBI is devoid of any test signal. Thus, in-service testing is limited to (1) synchronization pulse amplitude test and (2) chrominance burst amplitude test which basically confirm the presence or absence of video. These tests are incapable of measuring or assessing video quality.
The MPEG-2 video encoder limitation also encumbers broadcasters which often seek to insert test signals into the VBI at the point of signal origination to test end-to-end video quality. If their video transmission undergoes MPEG-2 to compression at any stage, the VBI test signals are lost.
Prior art related to in-service testing of video transmission systems includes:
U.S. Pat. No. 5,617,148 issued Apr. 1, 1997, filed Sep. 18, 1992 (Montgomery), discloses a controlled element for spectrum attenuation or a controlled filter used to aid in the insertion of a secondary signal into a video signal without distorting the blanking intervals or closed caption data contained in the blanking interval of the video signal.
U.S. Pat. No. 4,969,041 issued Nov. 6, 1990 (O'Grady), discloses data embedded in a video signal by adding a low level waveform to the video signal. The low level waveform has a level below the noise level of the video signal and corresponds to the data. To detect the data embedded in the video signal, the video signal is correlated with low level waveform corresponding to the data to produce a correlation coefficient. A high correlation coefficient indicates a presence of a low level waveform which is converted into data. The low level waveform extends over many video lines so that it does not occur at or near the same location within a video frame for many frames to avoid fixed pattern noise anomalies that may be detected by a viewer.
U.S. Pat. No. 5,585,858 issued Dec. 17, 1996, filed Apr. 15, 1994 (Harper), discloses a system for simulcasting a fully interactive program with a normal conventional program in the same standard video signal bandwidth. Unused lines of the video are preferably used for embedding additional interactive response audio channels and graphics and control data. Alternatively, interactive audio segments are provided either serially or one after another in the audio subcarrier or in cable frequency guardbands or pre-stored in memory at the interactive program box. More audio and graphics can be provided through the use of an external storage device or game cartridges. The additional data is entered at designated trigger points in the system through the use of overlaid logic sent down in embedded codes in the signal or resident in the software at the receiver location.
U.S. Pat. No. 5,572,247 issued Nov. 5, 1996, filed Jun. 14, 1991 (Montgomery), discloses signal processors for permitting the transparent reception of a data signal in the video bandwidth of a cable television system. The received signal has video and data components that are frequency interleaved in the video bandwidth in the active video interval. The data signal is modulated with a carrier at a non-zero multiple of a horizontal scanning rate of the video signal. The receiver selects a forward channel transmitting the combined signal responsive to a control signal and extracts the data portion of the transmitted combined signal.
U.S. Pat. No. 5,557,333 issued Sep. 17, 1996, filed Feb. 24, 1994, and U.S. Pat. No. 5,327,237 filed Jun. 14, 1994 (Jungo), discloses signal processes for permitting the transparent, simultaneous transmission and reception of a secondary data signal with a video signal in the video band. The signal processor in the transmitter rasterizes the data at the horizontal scanning rate and modulates the data with a data carrier at a non-integral multiple of the horizontal scanning rate to obtain frequency interleaving. The data is transmitted during the active video portion of each video line.
U.S. Pat. No. 5,663,766 issued Sep. 2, 1997, filed Oct. 31, 1994 (Sizer), discloses a system for communicating digital information in a video signal and comprising an encoder arranged to add a carrier signal modulated by digital information to the video signal. The modulated carrier signal at other than a frequency corresponds to a peak in the video spectrum. A receiver is arranged to optically sense the video signal and to recover the encoded digital information in the video signal.
In view of the prior art, a need exists to provide in-service testing of video transmission subject to the MPEG-2 compression algorithm where a video test signal may be inserted within the VBI or overscan or active area of a video frame for measuring or assessing video quality in a manner transparent to a viewer.